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FIX Client Simulator User Guide

FIX Client Simulator User Guide

Jan 23, 2026

Product Overview

EPAM B2BITS FIX Client Simulator (FCS) is a rich desktop UI application. It allows to simulate multiple FIX protocol-defined buy-side- and sell-side-oriented workflows and create repeatable test scenarios for FIX sessions. FCS combines the ability to generate, manually enter, and edit FIX messages in row format and via pre-defined forms. It allows to assemble and replay FIX messages in a highly automated fashion with automatic handling of sequence numbers and identities.

FCS is used in the following use cases:

  • OMS/EMS functional and stress testing

  • Automated FIX integration testing 

  • FIX protocol implementations

  • Errors reproducing

FCS covers all spectrum of FIX protocol versions and supports custom extensions and QuickFIX-formatted dictionaries.

FIX Client Simulator features

  • Establishing multiple acceptor and/or initiator sessions on local and/or remote hosts

  • Support of unregistered acceptor sessions

  • Restarting the working session with or without session logs restoration

  • Restoring session state and configurations on FCS startup

  • Support of all FIX message types (pre-trade, trade, post-trade, market data, etc.)

  • Standard 4.0 - FIXLatest FIX application versions support

  • Custom dictionaries support

  • Overriding SenderCompID and TargetCompID for the established session

  • Manual sequence numbers definition or reset

  • Validation of incoming and outgoing FIX and QuickFIX messages 

  • Wide range of FIX session settings including Custom Logon message

  • Replaying selected log files

  • Ability to load, modify, and send a file containing a preset message or batch of messages

  • Enabling/disabling message re-sending

  • Sending messages in a defined interval

  • Tag values autoincrement

  • Events monitoring:

    • sent and received messages

    • session states

  • Creating, recording, editing, and running test scenarios for FIX sessions

Prerequisites and System Requirements

FIX Client Simulator is based on FIX Antenna C/C++/.NET FIX Engine. It can be installed separately or as a part of the FIX Antenna .NET package.  

  • Any modern Intel-based server or workstation with a 64-bit CPU can be used as hardware.

  • Supported operating systems:

    • Windows 7

    • Windows 8

    • Windows 10

    • Windows 11

    • Windows Server 2008 R2

    • Windows Server 2012

    • Windows Server 2016

    • Ubuntu Linux 20.04

    • Ubuntu Linux 22.04

    • Rocky Linux 8

    • macOS

  • (For Windows users only) Latest (14.42.34438.0) Microsoft Visual C++ Redistributable Package (x64) should be installed before the FIX Client Simulator's installation.

  • (For Linux and macOS users only) Download and install Wine from the official site to run the FIX Client Simulator.

Installation on Linux Mint and Ubuntu

FCS has been available on Linux since the FIX Client Simulator 3.5.0 release.

To install FIX Client Simulator on Linux Mint and Ubuntu follow the instructions below:

  1. Install Wine (instruction for Ubuntu and Mint).

  2. Install Winetricks using the following commands:

    sudo apt install winetricks

  3. Install .NET Framework 4.8 using the following commands: 

    winetricks --force dotnet48 winetricks --force corefonts

    If Wine prompts for installing Mono, click Cancel. Please refer to the HowTo section for more details.

  4. Navigate to the FCS folder and run it.

    cd ./FCS_<version> wine Fcs.exe

Installation on macOS

FCS has been available on macOS since the FIX Client Simulator 3.5.0 release.

To install FIX Client Simulator on macOS follow the instructions below:

  1. Install Wine

    1. Option 1 - Download and install Wine from the official site

    2. Option 2 - Download and install Wine using Brew

      brew install --cask wine-stable

  2. Navigate to the FCS folder and run it.

    1. Option 1 - via Terminal

      cd ./FCS_<version> wine Fcs.exe

    2. Option 2 - via context menu

In addition, you can use modern Wine wrappers like Whisky. Whisky has a modern UI and active community support.

Configuration

FIX Client Simulator can be configured in the Fcs.exe.config file via the following properties:

Property Name

Description

Required

Default Value

Property Name

Description

Required

Default Value

DaysToKeepEventViewLog

This property is available since the FIX Client Simulator 3.3.0 version.

This property defines the number of days for which the session logs in Events Viewer will be restored during the logs restoration process.

Possible values: int

N

0

MainWindowTitle

This property is available since the FIX Client Simulator 3.6.0 version.

This property defines the name of the main FCS application window. If not defined then the default 'EPAM® FIX Client Simulator' title will be displayed.

Possible values: string

N

null

StoreSettingsInUserFolder

This property is available since the FIX Client Simulator 3.2.0 version.

This property defines whether the user.config file will be stored in the current user directory.

Possible values: true | false

N

false

TestScenarioFolder

This property is available since the FIX Client Simulator 3.7.0 version.

This property defines the path to the system directory where created Test Scenarios will be stored.

When the TestScenarioFolder property is defined with the default 'null' value all created Test Scenarios must be stored in the default TestScenarios folder within the application package.

Possible values: string

N

null

FIX Session creation

To create a session, choose: Session → Create new FIX session...:

Specify needed parameters in the Create Session window. When configuration is done, click “OK”. After the session is created, you will see it in the Session List section. All received messages will be displayed in the Events Viewer section below.

Session parameters

The left column of the Create Session window includes all required parameters. If at least one of them is specified incorrectly, the session will not be created.

The right column includes extended parameters that are optional.

Required parameters

Session type: Defines a type of the created session – Initiator or Acceptor.

SenderCompID: Key parameter for purposes of session identification. Sets value of SenderCompID (49) tag in outgoing FIX messages.

TargetCompID: Key parameter for purposes of session identification. Sets value of TargetCompID (56) tag in outgoing FIX messages.

FIX Version: Drop-down list of all available protocol versions, including custom parsers, if configured.

Remote host: Defines a network IP address of the computer, to which connection is established (for initiator).

Remote port: Defines a port's network number on the computer, to which connection is established (for initiator).

Heartbeat interval: Defines the heartbeat interval (seconds) – a time interval between Heartbeat messages

InSeqNum: Defines the initial incoming sequence number. It is expected that the first incoming message will have the specified sequence number.

If set to -1 and SeqNum reset is not enabled then SeqNum will be defined based on existing session logs

OutSeqNum: Defines the initial outgoing sequence number. The first outgoing message will be sent with the specified sequence number.

If set to -1 and SeqNum reset is not enabled then SeqNum will be defined based on existing session logs.

Use custom logon message: If enabled, allows to use custom logon, which should be entered to corresponding field.

Persistent storage type: If enabled, incoming and outgoing messages will be persisted in log files. If disabled, messages will be stored in memory and sessions' state won't be restored after FCS restart.

Reset SeqNum: Allows to select sequence numbers reset strategy. Available options: Reset SeqNums, Don't Reset SeqNums, Don't Reset SeqNums with 141=N.

Extended parameters

Using of extended session parameters can be enabled by checking "Use Extended properties" check-box.

SenderSubID: Sets value of SenderSubID (50) tag in outgoing FIX messages

TargetSubID: Sets value of TargetSubID (57) tag in outgoing FIX messages

SenderLocationID: Sets value of SenderLocationID (142) tag in outgoing FIX messages

TargetLocationID: Sets value of TargetLocationID (143) tag in outgoing FIX messages

Qualifier: Allows setting the name of SessionQualifier.

SourceIPaddress: Defines the expected value of the sourceIP address. If the real value is not equal to the expected one then the session is disconnected without sending a message and an error condition is generated in the log output (for acceptor).

UserName: Sets the value of the Username (553) tag in the Logon message.

Password: Sets the value of the Password (554) tag in the Logon message.

FAST templates: Defines the path to file with FAST templates. If specified, session will be created as FAST session.

Encryption: List of available encryption techniques to mask the application messages.

IntradayLogoutToleranceMode: Defines using of the Intraday Logout Tolerance mode in the session.

ForceSeqNumReset: Defines using the Force Sequence Number Reset mode for the session.

ForcedReconnect: Applied to initiator session. In case there is no acceptor exists (FCS is unable to establish connection), this property works with the 'Reconnect.MaxTries' property in 'engine.properties':

  • if ForceReconnect = false, the parameter Reconnect.MaxTries will be ignored, session does not try to reconnect;

  • if ForceReconnect = true, the parameter Reconnect.MaxTries will be used, session will try to reconnect 'Reconnect.MaxTries' times.

EnableMessageRejecting: When set to 'true', the session rejects application messages when the session is unable to send them during a specified period or was disconnected.

IgnoreSeqNumTooLowAtLogon: Defines how the FCS should resolve ‘Sequence number is too low’ problem at a logon message. When enabled the session ignores error and continues to work with received sequence number.

BackupConnectionParameters

The FIX Client Simulator 3.10.0 release introduces the "Backup Connection Parameters" section, which allows configuring a backup connection to ensure a seamless transition if the primary connection fails.

Host: Defines a network IP address of the computer, to which backup connection is established (for initiator).

Port: Defines a port's network number on the computer, to which backup connection is established (for initiator).

EnableCyclicSwitchToBackupConnection: If it is set as "true", a backup session can automatically switch back to the active session once the backup connection has problems.

EnableAutoSwitchToBackupConnection: If it is set as "true", an active session can automatically reconnect to its backup session in case there is a problem to connect to host/port of the active one.

KeepConnectionState: If enabled, the same log files for both connections are used. If disabled, separate logs will be created for the backup connection

SSL configuration

Using SSL can be enabled by checking "Use SSL" check-box.

SSL Protocols: List of supported SSL protocol versions. Allows to select needed versions to be used.

Ciphers: Comma-separated list of ciphers to use. The default OpenSSL built-in configuration is used in case of parameter absence.

Certificate: Path to SSL certificate.

Private Key: Path to SSL private key. If it is omitted Engine tries to load the private key from the same file as SSLCertificate parameter states.

Validate Peer Certificate: If set to "true" then connection counter-parties without a certificate are rejected. SSLCACertificate parameter must be specified.

CA Certificate file: This is a path to the file containing CA certificates.

Kafka Session creation

To create a session, choose:  Session → Create new Kafka Session...:

Specify needed session parameters in the Create Kafka Session window and click “OK”. After the Kafka session is created you will see it in the Session List section. All sent or received messages will be displayed in the Events Viewer section below.

Kafka session parameters

Main parameters

Session type: Defines a type of the created session.

Remote host: Defines a network IP address of the Kafka broker.

Remote port: Defines a port's network number of the Kafka broker.

Topic: Name of a topic to send or consume messages.

Key: A Key attached to each message sent from Producer session.

Group: A Group to which created consumer session relates.

From: Partition Offset for created Consumer session.

Authentication parameters

Mechanism: Defines authentication mechanism used for connection (None, SASL/PLAIN)

Username: Available if SASL/PLAIN authentication method is used. Sets the value of the Username used to establish connection.

Password: Available if SASL/PLAIN authentication method is used. Sets the value of the Password used to establish connection.

SSL parameters

Using of SSL can be enabled by checking “Enable SSL” check-box.

Client key: Path to SSL private key.

Client certificate: Path to SSL certificate.

Validate broker certificate: If enabled, two-way TLS is used.

Certificate Authority: Path to the file containing CA certificates.

Multiline session display

The FIX Client Simulator 3.9.0 update allows to view sessions with multiple lines within the Session list. This view can be accessed by following these steps:

  • Go to the Options menu

  • Select the Multiple session information

After enabling "Multiple session information" in the Options menu, the details for each session in the list will now be displayed across multiple lines.

This provides a clearer view of all the information for each session

 

Manual sequence numbers definition and reset

To define sequence numbers manually, open the Create Session window and specify values in InSeqNum and OutSeqNum input fields:

To reset sequence numbers manually open the Create Session window and tick Reset Sequence number.

Display administrative messages in the console

To see or hide administrative messages in the console open the Create Session window and select the Show session messages checkbox.

Alternatively, you can select a session and select the Show session messages option in the Session menu or in the context menu of the session window.

 

This setting is stored as other session settings and restored after the session restart.

Sending messages

In order to send a message, please specify the message in the Send Message tab, choose a session, and press the Send all or Send current and go Next button. Please note that the sequence number and sending time will be set automatically.

For Kafka session, you can send any text you need, including arbitrary content such as JSON; tags overriding is not available for such messages.

To override SenderCompID and TargetCompID select the Override SenderCompIDs or Override TargetCompID option in the main application menu in the Options section.

To specify the message-sending rate and count (number of messages to be sent at once) enter the value in the Rate and Send count input fields on the Send Message tab.

Message re-sending

To enable message re-sending open Options from the context menu and select the Allow message re-sending option.

Sending selected messages

This feature is available since the FIX Client Simulator 3.0.0 release.

In order to send message(s), open the Send Message tab, choose a session, and type the message(s) in the text box. To send all messages specified in Send Message tab, press the Send all button.

To send only selected message(s) select it in the Send Message tab and press Send Selected button. Only message(s) starting with the 8 tag and ending with the 10 tag will be sent.

The sent message will be highlighted in light grey.

Sending messages one by one

To send messages one by one select the message in the Send Message tab and press the Send Current and go Next button. The first sent message will be highlighted in light grey and the next message will be highlighted in dark grey.

To reset the selection of messages, please press the Reset button.

Autoincrement tag values

The feature works with a FIX or Kafka session

The ability to set the value starting from which the value of the tag will be autoincremented is available since FIX Client Simulator 3.1.0 release.

When sending messages, the values of the specified tags can be automatically increased with each sending. Tag numbers that must be changed are set in the Autoincrement tags field. 

If you want to change the values of several tags, the tag numbers must be separated by a space, e.g. '11 38'.

If the tag is included in the Repeating group, the root tag must go first then the '/' symbol, and then the particular tag number, e.g. '453/448'.

If you want to set the value starting from which the value of the tag must be autoincremented, the tag and its value must be separated by the '=' symbol, e.g. '11 = 100' or '58 = "Hello 22"' - for value with the space or '58="Hello \" 22"' - for value with the space and " symbol.

Starting from FIX Client Simulator 3.2.0 release autoincrementation can be reset only by changing the value in the Autoincrement tags field.

Possible Autoincrement Scenarios:

  1. String Values

    1. If the string concludes with a numeric value, the autoincrement operation increases this number by one. For example, if the initial value is "Test1", autoincremented values will be "Test2", "Test3", and so forth.

    2. If the string ends with any non-numeric character, the value "1" is appended to the string, and subsequent increments follow numerically. For instance, starting from "Test," the autoincremented values will be "Test1", "Test2", etc.

  2. Numeric Values

    1. For Integer values, the autoincrement operation increases the value by one. For example, an initial value of "10243" will be followed by "10244," "10245," and so on.

    2. For floating-point (double) values, the increment is determined by the smallest precision of the value. For example, 0.5 will be incremented by 0.1 (resulting in 0.6, 0.7, etc.), while 0.15 will be incremented by 0.01 (resulting in 0.16, 0.17, etc.).

  3. Time Values

    1. For time fields, the current time in UTC is applied.

    2. For DateTime fields, the current DateTime in UTC is applied.

Replay log

The feature works with a FIX or Kafka session

The ability to replay QuickFIX-formatted log files is available since FIX Client Simulator 3.1.0 release.

To send messages from the log file to a session (replay log file), please specify the log file in the Replay tab, choose a session, and press the Play button.  

Log files of the following formats may be replayed:

  • *.in

  • *.out

  • *.fix

  • *.msg

  • *.body

Also provides the ability:

  • to suspend sending messages from the log (Pause button)

  • to stop sending messages from the log (Stop button)

  • to send messages one by one (just after Loading file, after Pause, after Stop. Next button)

Please note that the Replay function does not send Administrative messages (35=0, 35=A, 35=5, 35=3, 35=2, 35=4, and 35=2) by design.

To send a specific range of messages it is possible to specify a filter by Sequence numbers or/and types of messages (Filter). 

The log can be replayed several times. The number of replays is set by the Send count parameter.

The Send count parameter is ignored if messages are sent one by one

To specify the message-sending rate for message sending enter the value in the Rate field in the Replay tab. Also, the interval can be calculated based on the timestamp in the log file (Use the timestamps tick). 

In addition, the interval for sending messages is changed by setting a multiplier other than one (Speed parameter).

Events monitoring

The upper right part of the FIX Client Simulator window displays the message(s) to be sent.

The upper left part shows the list of active sessions.

The lower part is the Events Viewer section, where each record corresponds to one event and consists of the event time, name, and description.

The New Events Viewer interface described below is available since the FIX Client Simulator 2.28.0 release.

Events Viewer displays events and messages of the chosen session in the Session List section. If you choose several sessions, all events belonging to the sessions are displayed in the Events Viewer section.

Whole messages or only their parts can be copied from the Events Viewer section.

  • To copy the whole message with the timestamp it must be selected by highlighting content within a single line or selecting multiple full lines, right-clicked on and then the Copy action must be selected in the context menu.

  • To copy the FIX message without timestamp it must be right-clicked on and then the Copy FIX Messages action must be selected from the context menu.

  • To copy the part of the message this part must be selected/highlighted and right-clicked on and then the Copy action must be selected in the context menu.

Checkboxes on the bottom of the Event Viewer section allow you to adjust the completeness of the event display, including messages. You can show or hide timestamps, header, or trailer of a FIX message, application messages, and session messages.

Events and messages in the Events Viewer section can be filtered using the Filter events by text field. All special symbols are supported except '/'.

Incoming, outgoing, and console messages are highlighted with different colors for visual usability. The colors can be changed in the Fcs.exe.config file.

The Events Viewer section can be spliced into three windows (Incoming, Outgoing, and Console) by choosing the Split Window checkbox. No highlighting is applied in this mode.

It is possible to auto-scroll the events by clicking Events Viewer → Auto scroll in the main menu so that Events Viewer will display the latest events first. 

To clear the Events Viewer section select the Clear event viewer option from the Events Viewer menu.

Grid view